Since inkjet recording methods have advantages over other image recording methods such that the process is simple; full color images can be produced relatively easily; and high resolution images can be produced even when an apparatus having a simple structure is used, inkjet recording methods become prevalent.
Inkjet recording methods typically include ejecting ink droplets using an air bubble generated in the ink by heat, a pressure generated in the ink by piezoelectricity, or an electrostatic force, so that the ink droplets are adhered to a recording medium such as paper sheets; and then quickly drying the ink droplets (or allowing the ink droplets to penetrate into the recording medium) so that the ink droplets dry, resulting in formation of an ink image on the recording medium. Thus, inkjet recording methods are simple, and can be used not only for personal purposes but also for industrial purposes such as printers and printing machines.
With expansion of industrial applications of inkjet recording, a need exists for an inkjet recording apparatus which can form images on various recording media (such as various papers) at a high speed. Specifically, a need exists for an inkjet printer having a line recording head to form images at a high speed. In addition, an increasing need exists for an inkjet printer, which uses an aqueous ink from environmental and safety standpoint.
However, image formation using an aqueous ink is considerably affected by the recording medium used, and various image problems are caused thereby. Particularly, when a plain paper having no coat layer on the surface thereof is used as a recording medium, various problems are caused. Since aqueous inks have a long drying time while having good affinity for paper, aqueous inks easily penetrate into recording paper. Particularly, when an aqueous ink image is formed on an uncoated paper having a rough surface, the colorant included in the aqueous ink penetrates into the paper, thereby causing a problem in which a low density image is formed and which is not caused when a solvent ink is used.
Particularly, when high speed image formation is performed by using an aqueous ink, a penetrant is typically included in the aqueous ink to shorten the drying time of the ink adhered to a recording medium. However, when a penetrant is included in an aqueous ink, not only penetration of water into recording media but also penetration of the colorant included in the aqueous ink into recording media is enhanced, thereby producing images having lower image density than in a case where the penetrant is not used for the aqueous ink.
In order to perform high speed image formation, it is preferable that change of the properties of the ink is small even when the environmental conditions change, or after long repeated use (i.e., preservation stability of the ink is good), and defective images such as omissions and white streak images, which are caused by clogging of nozzles of an inkjet recording head with the ink when the ink is ejected from the nozzles, are not formed (i.e., ejection stability of the ink is good).
In particular, since pigments are typically hydrophobic, aqueous inks including a pigment tend to have poor preservation stability. Therefore, a dispersant is typically used for aqueous inks including a pigment to improve preservation stability of the inks. However, the preservation stability of such aqueous inks is not yet complete. In addition, since resistance of aqueous inks to nozzles of an inkjet recording head is large, ejection stability of aqueous inks is not yet complete.
JP-H11-199805-A discloses an ink including a polycarbonyl compound and a semicarbazide derivative. In addition, a compound including diacetone acrylamide and methacrylic acid, in which the content of diacetone acrylamide is not greater than 20% by weight, is disclosed thereby.
JP-2004-9463-A discloses a pretreatment liquid including a saponified material of a copolymer of diacetone acrylamide-fatty acid vinyl ester, and an ink including a hydrazine compound. In this regard, the hydrazine compound included in the ink is reacted with the saponified material of a copolymer of diacetone acrylamide-fatty acid vinyl ester in the pretreatment liquid so as to crosslink, so that the ink is improved in properties such as bleeding property, water resistance, and rub resistance.
JP-2006-16458-A discloses an ink including a pigment covered with a resin having a N-substituted methacrylamide unit and a carbonyl group. In addition, the ink can further include an optional hydrazine compound. Diacetone acrylamide is exemplified as a monomer having a carbonyl group, but the content thereof is not greater than 50%.
The ink of JP-H11-199805-A is proposed to improve water resistance and fixability thereof. However, as a result of the present inventors' investigation, the ink has insufficient ejection stability and preservation stability, and images produced by the ink have low image density. In addition, the ink of JP-2004-9463-A has insufficient ejection stability and preservation stability, and images produced by the ink have low image density. In addition, since the resin in the ink of JP-2006-16458-A, which has a N-substituted methacrylamide unit and a carbonyl group, has both a cationic group and an anionic group, the ink tends to easily agglomerate. Therefore, the ink has insufficient ejection stability and preservation stability, and images produced by the ink have low image density.